Mystery at the Manor

Situation
Situation puzzles (sometimes called lateral thinking puzzles) are ones where you need to ask lots of yes or no questions to figure out what happened in the situation. These are good puzzles for groups where one person knows the puzzle and answers the questions.

You are detective Sherlock Holmes.You have been assigned to a murder case.
There's a scream in Buckingham Manor, and Sir Buckingham has been murdered. He lived there with a fortune in his will for the only people he knew...Cadaver, his cook...Corpse, his butler...and Creeper, his groundskeeper. Of course, now you should expect one of the three to be the murderer. Here are the clues for you to find out the murderer:

1)Cadaver always suspected Creeper because he always sneaked around.
2)Creeper suspected Corpse because he was always suspicious of the butler for trying to steal Sir Buckingham's ruby.
3)Corpse suspected Cadaver because he always saw her putting something very strange in Sir Buckingham's soup.

Answer

It could only be Cadaver, the cook.
She is spotted trying to put something in Sir Buckingham's soup, and why would she worry about Creeper sneaking about if she might not have been doing anything wrong?
As soon as she found out about Corpse stealing the ruby, she decided to steal it for herself. Creeper saw her at the time of the burglary and the time of the murder. Sneaking about in the garden is sort of his job.